1. Technical Field
The present invention relates to a parabolic antenna for use in satellite broadcasting or satellite communication and more particularly, to a primary radiator and a block-down-converter which constitute the parabolic antenna.
2. Background Art
Conventionally, parabolic antennas which receive radio waves from a plurality of stationary satellites by a single reflector are referred to as xe2x80x9cdual-beam antennasxe2x80x9d or xe2x80x9cmulti-beam antennasxe2x80x9d and are mainly adapted to receive radio waves from two satellites located on a stationary orbit with a difference of longitude of 8 degrees.
One example of the parabolic antennas is proposed in Japanese Utility Model Laid-Open Publication No. 3-107810 (1991) and FIG. 27 is a perspective view showing its arrangement. In FIG. 27, a dual-beam antenna 100 includes primary radiators 102 and 103 constituting a double primary radiator and a reflector 101. The primary radiators 102 and 103 and the reflector 101 are coupled with each other by a support arm 104 so as to have a predetermined positional relationship. Radio waves from first and second satellites are reflected by the reflector 101 so as to be, respectively, received by the primary radiators 102 and 103. In this dual-beam antenna, axes of the primary radiators are disposed so as to extend horizontally at the time of reception.
Meanwhile, circular polarization is employed as polarization in satellite broadcasting, while linear polarization of two kinds, i.e., in vertical and horizontal directions is employed as polarization in satellite communication. Therefore, radio waves from a communication satellite contain a polarization angle dependent on a receiving point and thus, this polarization angle should be adjusted.
A method of adjusting the polarization angle is proposed in Japanese Utility Model Laid-Open Publication No. 6-52217 (1994). FIG. 28 is a perspective view indicative of one example of adjustment of the polarization angle. As shown in FIG. 28, the adjustment is performed by rotating an arm 113 through an angle xcex8b about an axis of a fixed primary radiator 111 and further, rotating a primary radiator 112 through an angle xcex8a about its own axis.
FIG. 29 a relationship between antenna diameter D and primary radiator spacing L in the case where difference of longitude between two satellites on a station orbit is 8 degrees and 4 degrees. As shown in FIG. 29, the reflector diameter D and the primary radiator spacing L are substantially proportional to each other and an optimum value of the primary radiator spacing at the time the difference of longitude is 4 degrees is smaller than that at the time the difference of longitude is 8 degrees.
FIG. 30 shows a relationship between aperture diameter d of a primary radiator and antenna efficiency xcex7 in a single-beam antenna. As shown in FIG. 30, when the aperture diameter d assumes dopt, the antenna efficiency xcex7 reaches a maximum xcex7max as follows. If the aperture diameter is small, radiation range over the reflector increases and thus, energy of the reflector spills from the reflector, namely, spill-over happens. On the other hand, if the aperture diameter is large excessively, radiation range decreases and thus, an edge portion of the reflector does not work.
Therefore, in case a dual-beam antenna for receiving radio waves from two satellites with a difference of longitude of 4 degrees is formed by using an antenna having a diameter Do and primary radiators having an optimum aperture diameter dopt, the spacing Lo should be larger than dopt. As shown in FIG. 29, in case a dual-beam antenna is formed by using a reflector having a smaller effective diameter Ds, the spacing L decreases to Ls. If the spacing Ls is smaller than dopt, the aperture diameter d necessarily becomes smaller than dopt yielding the maximum efficiency xcex7max, so that the antenna efficiency xcex7 drops markedly to xcex7o as shown in FIG. 30 and thus, it becomes difficult to obtain desired reception performance.
In order to obviate the above mentioned drawbacks, a double primary radiator of the present invention has a construction in which by using a small-diameter parabolic reflector having an effective diameter of, for example, 45 cm, two primary radiators are integrally joined with each other so as to receive radio waves from two satellites having a difference of longitude of, for example, 4 degrees.
In the double primary radiator of the present invention, since apertures of the primary radiators are arranged to face each other inwardly, it is possible to compensate for reduction of radiation area due to defocus caused in the case where a dual-beam antenna is arranged such that a central point of a joint part of the double primary radiators is located in the vicinity of a focal point of the reflector.
Since a block-down-converter of the present invention can be rotated as a whole about a perpendicular radiation axis, tilt angle of the two radiators can be adjusted relative to polarization angle at a time.
In the block-down-converter of the present invention, if an initial shift angle for adjusting polarization angle is set to that of a point located substantially at a center of a longitudinal range of a receiving area, adjustment of the initial shift angle can be substantially optimized throughout the receiving area. Therefore, since it is not necessary to adjust the initial shift angle for each receiving point, the block-down-converters can be mass produced.
Meanwhile, since the block-down-converter of the present invention has a construction in which a double primary radiator and a housing containing a conversion circuit for performing amplification and frequency conversion of received radio waves are integrally molded, the block-down-converter can be produced by a simple process such as injection molding employing a die, thereby resulting in a reduced production cost.